Clock movements with a perpetual calendar are known. The purpose of these watches or clocks is automatically to give the date as well as the time. In very general terms, they are made to show the day, the month and the date according to the Gregorian calendar. Perpetual calendar watches automatically take into account the different lengths of the months and of leap years. The mechanisms of these perpetual calendar movements are based on the presence of a 48-tooth wheel which is actuated for changing the 48 months between two leap years. This wheel therefore performs one rotation in 4 years. Fixed to its upper surface is a cam divided into 48 steps; their arcs are equal but their depth varies according to the length of the month which they represent.
Clock movements with equation of time are also known. These equation-of-time watches or clocks possess a pair of hour and minute hands showing the mean or civil time, as in most watches. These watches also show the difference between the civil time and the true time, by means of an equation-of-time am.